[TTS] Updates file-level comments for this Feature

Adds or updates file-level comments for most of the key classes
used by Contextual Search.

There are no functionality changes. This CL consists only of file-level
comment updates.

Change-Id: I44a1ab287bea16cf383dfb2c1769f09171144eba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2487835


Reviewed-by: Theresa <twellington@chromium.org>
Reviewed-by: Sinan Sahin <sinansahin@google.com>
Reviewed-by: Jinsuk Kim <jinsukkim@chromium.org>
Commit-Queue: Donn Denman <donnd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820066}